AT&T introduced its rollover concept to data plans for Mobile Share Value customers to give an extension of one month to drain the unused data.

AT&T is in fighting mode. In response to T-Mobile's bold rollover data concept, AT&T said it will start giving a similar rollover data option to Mobile Share Value customers but with a few tweaks.

Set to go live on January 25, the new plan will give an extension of one month for unused data instead of confiscating it altogether at the end of the monthly cycle. But there are some caveats as pointed out by T-Mobile's flamboyant CEO John Legere.

AT&T will add its rollover data plan to all new and existing customers starting later this month, at no extra charge. Under the new change, AT&T's nearly 50 million Mobile Share Value subscribers will be able to move their unused data to next month and also share that data with their other accounts and devices. But if the data goes unused for the second month, it simply disappears.

For instance, if a customer has a 15GB data plan and uses only 10GB in a month, the remaining 5GB gets carried forward to the next month to be added with the new allotment. This means, subscribers will have a total of 20GB. But the rolled over data can be used only after the newly allotted data is exhausted.

In the case of T-Mobile, subscribers have the option to rollover their unused data allowance for up to 12 months. Even though T-Mobile's data rollover plan was introduced first, AT&T says its rollover concept for data is original and a spinoff from its voice rollover plan.

"It's just the opposite. I think they took our rollover concept and we're flattered that they decided to use it for data. We had plans to roll it out here at CES for a long time. We trademarked the name "Rollover Data" earlier in the year," Ralph de la Vega, the president and CEO of AT&T Mobility and Business Solutions, told USA Today.

AT&T's rollover data plan is applicable to all data plans, including the cheapest ones. But T-Mobile doesn't offer the same, as it excludes its cheapest Simple Choice Plan subscribers from its Data Stash rollover plan. That hasn't stopped T-Mobile's CEO from mocking AT&T with his colorful response on Twitter.